The ideas of Lagrange, Poisson, Kelvin and Truesdell are reviewed. It is shown that in order for a bounding surface not to be a material surface either u. n = c must fail or more than one deformation can be associated with the velocity field. Examples are given.

The linear stability of modulated circular Couette flow to axisymmetric disturbances is examined in the narrow-gap limit. The outer cylinder is assumed stationary, while the inner is modulated both with and without a mean rotation. The equations governing the disturbance motion are solved by a Galerkin expansion with time-dependent coefficients, and the stability of the motion determined by Floquet theory. Modulation is found, in general, to destabilize the flow due to steady rotation, although weak stabilization is found for some modulation amplitudes at intermediate frequencies.

A body is started impulsively from rest and moves on an arbitrary path in an incompressible, stably stratified, rotating fluid. The phase configuration of the waves which are generated is studied using small amplitude wave theory. The theory is compared with experiment for a few special cases which include a horizontal cylinder (a) oscillating about a position fixed in the fluid, (b) moving with constant velocity and (c) moving with a constant angular velocity in a circular path relative to the fluid. Theory and experiment show reasonable agreement except where wakes interfere with the wave pattern.

The diffusion of slowly varying, weak magnetic fields by a statistically isotropic and stationary velocity field in a perfectly conducting fluid is studied by Eulerian analysis. The characteristic wavenumber and variance of the velocity field are k0 and 3v20, thus defining the eddy-circulation time τ0 = 1/v0k0. The velocity field is assumed constant on intervals of duration 2τ1 and statistically independent for distinct intervals. Thus the correlation time is τ1. The α-effect dynamo mechanism in the quasi-linear approximation is corroborated. Both the quasilinear and the direct-interaction approximations give identical diffusion of magnetic and passive scalar fields in reflexionally invariant turbulence. This result is found to be exact for τ1/τ0 → 0 but is demonstrated to be incorrect in general for finite τ1/τ0 because of effects of helicity fluctuations. The nature of the failure of the direct-interaction approximation is exhibited by an exactly soluble model system. Analysis based on a double-averaging device shows that longrange, persistent helicity fluctuations in reflexionally invariant turbulence give an anomalous negative contribution to the magnetic diffusivity which depends on the helicity covariance function. We term this the α2 effect. The magnitude of the effect depends sensitively on the turbulence statistics. If the characteristic scales of the helicity fluctuations are sufficiently larger than τ0 and 1/k0, the magnetic diffusivity is negative, implying unstable growth, while a passive scalar field diffuses normally. On the other hand, a crude estimate suggests that the α2 effect is small in normally distributed turbulence.

The problem of a uniform transverse flow past a prolate spheroid of arbitrary aspect ratio at low Reynolds numbers has been analysed by the method of matched asymptotic expansions. The solution is found to depend on two Reynolds numbers, one based on the semi-minor axis b, Rb = Ub/v, and the other on the semi-major axis a, Ra = Ua/v (U being the free-stream velocity at infinity, which is perpendicular to the major axis of the spheroid, and v the kinematic viscosity of the fluid). A drag formula is obtained for small values of Rb and arbitrary values of Ra. When Ra is also small, the present drag formula reduces to the Oberbeck (1876) result for Stokes flow past a spheroid, and it gives the Oseen (1910) drag for an infinitely long cylinder when Ra tends to infinity. This result thus provides a clear physical picture and explanation of the ‘Stokes paradox’ known in viscous flow theory.

We have applied the methods of classical statistical mechanics to derive the inviscid equilibrium states for one- and two-layer nonlinear quasi-geostrophic flows, with and without bottom topography and variable rotation rate. In the one-layer case without topography we recover the equilibrium energy spectrum given by Kraichnan (1967). In the two-layer case, we find that the internal radius of deformation constitutes an important dividing scale: at scales of motion larger than the radius of deformation the equilibrium flow is nearly barotropic, while at smaller scales the stream functions in the two layers are statistically uncorrelated. The equilibrium lower-layer flow is positively correlated with bottom topography (anticyclonic flow over seamounts) and the correlation extends to the upper layer at scales larger than the radius of deformation. We suggest that some of the statistical trends observed in non-equilibrium flows may be looked on as manifestations of the tendency for turbulent interactions to maximize the entropy of the system.

Slow flow of a viscous incompressible fluid past a slender body of circular crosssection is treated by the method of matched asymptotic expansions. The main result is an integral equation for the force per unit length exerted on the body by the fluid. The novelty is that the body is permitted to twist and dilate in addition to undergoing the translating, bending and stretching, which have been considered by others. The method of derivation is relatively simple, and the resulting integral equation does not involve the limiting processes which occur in the previous work.

When Rayleigh-Bénard convection is generated under random conditions, the finite amplitude rolls and bimodal flow are observed to possess randomly placed dislocations where the rolls fit together poorly. The dislocations move into the small wavelength convection, and hence provide a size-adjustment mechanism. It is observed that the dimensionless speed of the movement is smaller for larger Prandtl number fluid.

A flow-visualization study has shown that strong Kármán vortices develop behind the blunt trailing edge of a plate when the free-stream velocities over both surfaces are equal and that the vortices tend to disappear when the surface velocities are unequal. This observation provides an explanation for the occurrence and disappearance of certain discrete tones often found to be present in the noise spectra of coaxial jets. Both the vortex formation and the tones occur at a Strouhal number based on the lip thickness and the average of the external steady-state velocities of about 0.2.

Results from theoretical calculations of the vortex formation, based on an inviscid incompressible analysis of the motion of point vortices, were in good agreement with the experimental observations.

Experiments reported previously determined the detention time of airborne smoke particles momentarily trapped in the wake bubble behind a flat disk normal to smooth air flow. The dimensionless group H, the product of the detention time td and mainstream air velocity U divided by the disk diameter D, was found to be 7.44 for all combinations of U, D and the Reynolds number, a result that was consistent with a suggested physical model for particle transport across the bubble boundary. The work is now extended into the regime of turbulent free-stream flow, where H is seen to decrease with an increasing level of turbulence while the base pressure coefficient becomes more negative. At the same time, the length of the bubble decreases, as does the bubble shape factor (the ratio of bubble volume to surface area, non-dimensionalized with respect to D). A simple theoretical relationship between H and the base pressure coefficient is argued, and is found to be in good agreement with experiment.

An important conclusion from this work is that the free-stream turbulence parameter $\Lambda \equiv l_fk^{\frac{1}{2}}_f/DU $ (where lf and kf are the length scale and the kinetic energy of the free-stream turbulence respectively) controls the properties of the flow about the disk.

This work has potential applications in several areas of topical technological interest.

The shock wave equations for a perfect gas often provide more than one solution to a problem. In an attempt to find out which solution appears in a given physical situation, we present a linearized analysis of the equations of motion of a flow field with a shock boundary. It is found that a solution will be stable when there is supersonic flow downstream of the shock, and asymptotically unstable when there is subsonic flow downstream of it. It is interesting that both flows are found to be stable against disturbances of the d'Alembert type which grow from point sources; it is only when larger-scale line sources are considered that one can discriminate between the stabilities of the two types of flow. The results are applicable to supersonic flow over flat plates at incidence, to wedges, and to some cases of regular reflexion, diffraction and refraction of shocks.

As a model of an almost fully extended macromolecule, a small, flexible, inextensible, nearly straight thread in a shearing flow with weak Brownian motions is considered. The hydrodynamic resistance to motion is included using the slenderbody theory for Stokes flow. The variation of the small transverse displacements along the thread is expressed as a truncated sum of Fourier components, with appropriately chosen modal functions. A diffusion equation is derived in the Fourier space and solved. The expected deformation of the thread is then given for axisymmetric and two-dimensional straining flows. The transverse displacement of the ends and the small shortening of the projected length of the thread are both found to be sensitive to the truncation of the Fourier representation, although it becomes clear on physical grounds that the ratio of the shortening to the typical transverse distortion should increase with the number of degrees of freedom. In simple shear flow the deformation increases as the thread aligns with the flow, until the analysis breaks down when the entire thread is no longer in the extensional quadrants. The influence of the 2:1 ratio of the resistance coefficients from the slender-body theory is found to be a small numerical factor.

The modified Oseen linearization of the swirling-flow boundary layer of a mature hurricane is discussed. Upper and lower solutions for the radial and azimuthal velocity components are constructed in a subdomain by using a comparison theorem. It is shown that the error incurred in these velocity components by using the linear solution is no more than 30% in the subdomain. It is then inferred that the vertical velocity is also approximated to that order.

A body whose boundary C is vertical at the free surface F oscillates at high frequency in some prescribed manner. Asymptotic forms in terms of limit potentials are obtained for the upward force and the moment about a line in F of the pressure forces on the body. The cylindrical waves radiated to infinity are found to be asymptotically equivalent to those obtained in a two-dimensional solution of the Helmholtz equation. Some illustrative examples are given, principally the horizontal ellipsoid, in which case comparison with strip theory is possible.