ALBERT

Description: Within the Western Ross Sea, there are six emperor penguin colonies of widely different size that occur exclusively on sea ice. In 1990 a survey of all six sites, two by close overflights and four from the ground, showed that the breeding habitats were highly variable. The most important physical characteristics of these habitats appear to be stable fast ice, nearby open water, access to fresh snow, and shelter from the wind.

Description: The groundmass of andesitic dykes at Sezaki, southwest Japan, has trachytic texture and contains microscopic shear zones. The shear zones comprise a conjugate pair formed by flattening of the solidifying dyke rock, probably caused by the magma pressure of the still molten part of the dyke. This pressure shortened the solidifying rock perpendicular to the dyke margins and caused it to extrude parallel to the magma flow direction. The groundmass shears indicate that locally the magma flowed 60° upward in the dykes. It is concluded that while groundmass shears are a useful indicator of flow direction in dykes, phenocryst alignment in dykes is strongly influenced by magma-pressure flattening and thus may be a poor indicator of flow direction.

Description: Outer membrane (OM), cytoplasmic membrane (CM) and intracytoplasmic membranes (ICM) from the halophilic phototrophic purple sulphur bacterium Ectothiorhodospira mobilis 9903 were purified and characterized. The three membrane fractions were significantly different in regard to protein profiles on SDS-PAGE, and to the composition of amino acids, fatty acids and lipids. The presence of lipoproteins, the occurrence of lyso-phosphatidyl-ethanolamine and an increased content of saturated and short-chain fatty acids are characteristic properties of the OM. CM and ICM fractions are different on the basis of buoyant density, of protein profiles and amino acid composition, and due to the presence of succinate dehydrogenase activity in CM. In addition, CM and ICM showed significant differences in pigment content and absorption spectra.

Description: The stomachsof 23 striped dolphins (Stenella coeruleoalba Meyen, 1833, Cetacea), stranded along the Ligurian coast (western Mediterranean Sea), contained 32 species of cephalopods, crustaceans and fishes, totalling an estimated 2,723 prey specimens representing about 36 kg in weight. Cephalopods and bony fishes were equally important in the diet (50%). Todarodes sagittatus (34.5%) and Micromesistius poutassou (25.9%) were found to be the most important food species. Other species belonging to six cephalopod families, three crustacean families and nine bony fish families, contributed to the diet with variable numbers, weights, and occurrences, demonstrating the opportunistic character of striped dolphin feeding.

Description: Samples of two loliginid squids Alloteuthis africana and A. subulata were collected from the continental shelf off the west Sahara in August-September 1987. Statoliths were taken from 124 specimens and processed using statolith ageing techniques. Statoliths of both species were very similar in shape. In the ground statolith, growth increments were examined and grouped into four growth zones distinguished mainly by the width of the increments. Age of adult mature males of both species did not exceed eight months, that of females six months. Alloteuthis africana grew faster than A. subulata in weight and, particularly, in length. At age 180 d the mantle of A. africana was twice as long and the body weight 1·2–1·5 times as large. Both species matured over a wide range of sizes and ages (from 120 to 180 d). The life span of A. africana and A. subulata hatching between January and May on the west Saharan shelf is about six months, much shorter than that of A. subulata in its northern temperate range.

Description: Statoliths of Loligo gahi were sampled in the fishery region 45–47°S on the Patagonian shelf during September 1989. Peculiarities of the growth zones in the ground statoliths of adults are described. Maximum age of large maturing and mature females (130–160 mm of mantle length, ML) was estimated to be 325–345 d, that of large mature males (250–290 mm ML) ranged from 360 to 396 d. The squid Loligo gahi d'Orbigny, 1835, occurs in temperate shelf and upper slope waters of the Atlantic and Pacific coasts of South America and is caught commercially by the international fleet in the southern part of the Patagonian shelf within the Falkland Islands Interim Conservation Zone (FICZ) (Roper et al., 1984; Csirke, 1987). Occasionally, dense shoals of L. gahi appear in the fishery region 45–47°S off the Exclusive Economic Zone of Argentina (EEZA) and have been caught in significant numbers by trawlers at depths of 120–150 m in September-October (Chesheva, 1990). Loligo gahi is a medium sized loliginid; in Falkland waters males attain 350 mm ML, females 210 mm ML (Hatfield, 1991), while in the fishery region 45–47°S maximum size is 260 mm and 160 mm, respectively (Chesheva, 1990). Patterson (1988) revealed two Falkland spawning stocks of L. gahi of unclear status, spring-spawners and autumn-spawners (austral seasons) and pointed out that the life span of squid of each stock lasted ~1 y. Recently Hatfield (1991) used statoliths to elucidate Patterson's (1988) estimations of age and growth of Falkland stocks of L. gahi and confirmed the 1-y duration of L. gahi's life span.

Description: A theoretical examination is made of convection, induced by applied thermal and solutal gradients inclined to the vertical, in a shallow horizontal layer of a saturated porous medium. The horizontal components of these gradients induce a Hadley circulation, which becomes unstable when the vertical components are sufficiently large. A linear stability analysis is carried out, and calculations are made using a low-order Galerkin approximation for the various modes of instability. The orientation of the preferred mode and the other critical quantities are determined for representative parameter values. © 1993, Cambridge University Press. All rights reserved.

Description: The origin and evolution of spatially stationary streamwise vortical structures in plane mixing layers with laminar initial boundary layers were recently examined quantitatively (Bell & Mehta 1992). When both initial boundary layers were made turbulent, such spatially-stationary streamwise structures were not measured which is indicative of the high sensitivity of these structures to initial conditions. In the present study, the effects of four different types of spanwise perturbations at the origin of the mixing layer were investigated. The wavelengths of the imposed perturbations were chosen to be comparable to the initial Kelvin-Helmholtz wavelength. For the first two perturbations, the boundary layers were otherwise left undisturbed. A serration on the splitter plate trailing edge was found to have a relatively small effect on the formation and development of the streamwise structures. The introduction of cylindrical pegs in the high-speed side boundary layer not only generated a regular array of vortex pairs, but also affected the mixing-layer growth rate and turbulence properties in the far-field region. For the other two perturbations, the initial boundary layers were tripped on the splitter plate. An array of vortex generators mounted in the high-speed boundary layer and a corrugated surface attached to the splitter plate trailing edge had essentially the same effects. Both imposed a regular array of relatively strong streamwise vortices in counter-rotating pairs upon the mixing layer. This resulted in large spanwise distortions of the mixing layer mean properties and Reynolds stresses. While the vorticity injection increased the growth rate in the near-field region as expected, the far-field growth rate was reduced by a factor of about two, together with the peak Reynolds stress levels. This result is attributed to the effect of the relatively strong streamwise vorticity in making the spanwise structures more three-dimensional and hence reducing entrainment during the pairing process. The imposed streamwise vorticity did not follow the pattern of increasing spanwise spacing seen in the ‘naturally occurring ’ streamwise vorticity. The mean streamwise vorticity decayed with increasing streamwise distance in all cases, albeit at different rates. © 1993, Cambridge University Press. All rights reserved.

Description: An envelope model is applied to the case of a two-dimensional channel with ciliated parallel walls. The formulation assumes identical values of the longitudinal and transverse amplitudes, frequency and wavelength of the two walls; it allows for arbitrary phase relations and arbitrary (not too small) spacing, and it includes an externally imposed pressure gradient. General results of a second-order perturbation analysis of creeping flow are presented. The time-averaged steady mean velocity may be viewed as the sum of two contributions: that of the pressure gradient (Poiseuille flow), and that of ciliary-driven motion which, owing to nonlinearities, also depends on the pressure gradient and reduces to pure streaming in the absence of a pressure gradient. For zero pressure gradient, the ratio of the streaming velocity of the channel and that of a single sheet shows the degree to which streaming is augmented or impeded by flow interaction. This ratio increases for the symplectic and peristaltic cases, but decreases for the antiplectic case, as the width of the channel decreases for fixed values of phase relation and amplitudes. The net flow arising from streaming and pressure gradient is shown as pump characteristics, and associated efficiencies are given. The results indicate that propulsion (pumping) is greatest and most effective for symplectic metachronism in ciliated channels with predominantly transverse waves, that it is nearly as good for peristaltic motion, but that it is considerably inferior for antiplectic metachronism in channels with predominantly longitudinal waves. © 1993, Cambridge University Press. All rights reserved.

Description: The present numerical simulation explores a thermal-convective mechanism for oscillatory thermocapillary convection in a shallow rectangular cavity for a Prandtl number 6.78 fluid. The computer program developed for this simulation integrates the two-dimensional, time-dependent Navier-Stokes equations and the energy equation by a time-accurate method on a stretched, staggered mesh. Flat free surfaces are assumed. The instability is shown to depend upon temporal coupling between large-scale thermal structures within the flow field and the temperature sensitive free surface. A primary result of this study is the development of a stability diagram presenting the critical Marangoni number separating the steady from the time-dependent flow states as a function of aspect ratio for the range of values between 2.3 and 3.8. Within this range, a minimum critical aspect ratio near 2.3 and a minimum critical Marangoni number near 20 000 are predicted, below which steady convection is found. © 1993, Cambridge University Press. All rights reserved.

Description: During the collapse of an initially spherical cavitation bubble near a rigid wall, a re-entrant jet forms from the side of the bubble farthest from the wall. This re-entrant jet impacts and penetrates the bubble surface closest to the wall during the final stage of the collapse. In the present paper, this phenomenon is modelled with potential flow theory, and a numerical approach based on conventional and hypersingular boundary integral equations is presented. The method allows for the continuous simulation of the bubble motion from growth to collapse and the impact and penetration of the re-entrant jet. The numerical investigations show that during penetration the bubble surface is transformed to a ring bubble that is smoothly attached to a vortex sheet. The velocity of the tip of the re-entrant jet is always directed toward the wall during penetration with a speed less than its speed before impact. A high-pressure region is created around the penetration interface. Theoretical analysis and numerical results show that the liquid—liquid impact causes a loss in the kinetic energy of the flow field. Variations in the initial distance from the bubble centre to the wall are found to cause large changes in the details of the flow field. No existing experimental data are available to make a direct comparison with the numerical predictions. However, the results obtained in this study agree qualitatively with experimental observations. © 1993, Cambridge University Press. All rights reserved.

Description: The paper is devoted to the creation of an original model of propagation of weak but finite-amplitude waves initiating an exothermic process connected with chemical reaction or relaxation of a non-equilibrium medium. The medium is single phase (a fluid), or a homogeneous two-phase medium (liquid with gas bubbles). A nonlinear differential model describing wave-kinetic interaction and wave evolution is derived. The linear dispersion and dissipative features of these systems are investigated both analytically and numerically. Attention is paid to explanation of the physical mechanisms resulting in the formation of a self-sustained solitary wave, which in terms of synergetics could be called a ‘dissipative structure’. © 1993, Cambridge University Press. All rights reserved.

Description: An extension of an earlier theory of the two-dimensional incompressible flow past an isolated body is described. For a crossflow cascade of bodies, each of unit size in the crossflow direction and distance 2H apart, the region of validity of the extended theory covers H ≫ 1. A comparison with recent numerical calculations is favourable and a tentative asymptotic structure for the case of H = O(1) is described. © 1993, Cambridge University Press. All rights reserved.

Description: The near-wall flow structure of a zero-pressure-gradient flat-plate turbulent boundary layer with a single-layer viscoelastic compliant surface was visualized using the hydrogen-bubble technique. The compliant materials were made by mixing silicone elastomer with silicone oil. The flow-visualization experiments indicated low-speed wall streaks with increased spanwise spacing and elongated spatial coherence compared to those obtained on a rigid surface. More interestingly, an intermittent relaminarization-like phenomenon was observed at low Reynolds numbers for the particular compliant surface investigated. Apparently, the observed changes in the near-wall flow structure over the compliant surface are caused by the stable interaction between the compliant surface and the turbulent flow-field. Optical holographic interferometry and laser Doppler velocimetry were also employed to obtain the basic parameters of the turbulent boundary layers and the flow-induced compliant-surface displacements to better understand the physics of the interaction between a turbulent boundary layer and a passive compliant surface. © 1993, Cambridge University Press. All rights reserved.

Description: We consider non-axisymmetric motion of a rigid particle in a cylindrical fluid-filled tube, with negligible inertial effects. The particle is assumed to fit closely in the tube, and lubrication theory is used to describe the fluid flow in the narrow gap between the particle and the tube wall. The solution to the Reynolds lubrication equation and the components of the resistance matrix are expressed in terms of a Green’s function. For the case in which the gap is almost uniform, the Green’s function is expanded as a power series in a small parameter 8, characteristic of the variations in gap width, and the first two terms are obtained. The velocity of a freely suspended axisymmetric particle driven by a pressure difference along the tube is deduced from the resistance matrix. According to the results at first order in 8, in general the particle moves transversely with a constant velocity. In the absence of higher-order effects, it would eventually collide with the wall. Motion along the tube axis is a neutrally stable solution to the equations of motion at first order. However, if effects at second order in δ are included, motion of an axisymmetric particle along the tube axis is stable or unstable depending on its shape. Generally, if the particle is narrower near the front than near the rear, and the width near the middle is at least as large as the mean of the widths near the front and rear, then its motion is stable. Numerical calculations (not restricted to small δ) confirm these results for axisymmetric particles, and show that a non-axisymmetric shape similar to a red blood cell has a stable equilibrium position in the tube. © 1993, Cambridge University Press. All rights reserved.

Description: It is known that stationary fluid with density that varies sinusoidally with small amplitude and wavenumber K in the vertical direction is unstable to disturbances that are sinusoidal in a horizontal direction with wavenumber a. Small values of α/K are the most unstable in the sense that a neutral disturbance exists at sufficiently small α/K however small the Rayleigh number may be. The non-uniformity of density in the undisturbed state may be regarded as being a consequence of non-uniformity of concentration of extremely small solid particles in fluid. This paper is concerned with the corresponding instability of such a non-uniform dispersion when the particle size is not so small that the fall speed relative to the fluid is negligible. In the undisturbed state, which is an outcome of the well-known primary instability of a uniform fluidized bed with particle volume fraction ϕ0, the sinusoidal distribution of concentration propagates vertically, and in the steady state relative to this kinematic wave particles fall with speed V (= |ϕdU/dϕ|ϕ0), where U(ϕ) is the mean speed of fall of particles, relative to zero-volume-flux axes, in a uniform dispersion with volume fraction ϕ. This particle convection with speed V transports particle volume and momentum and tends to even out variations of a disturbance in the vertical direction and thereby to suppress a disturbance, especially one with small α/K. Analysis of the behaviour of a disturbance is based on the equation of motion of the mixture of particles and fluid and an assumption that the disturbance velocities of the particles and the fluid are equal (as is suggested by the relatively small relaxation time of particles). The method of solution used in the associated pure-fluid problem is also applicable here, and values of the Rayleigh number as a function ofα/K for a neutral disturbance and a given value of the new non-dimensional parameter involving V are found. Particle convection with only modest values of V stabilizes all disturbances for which α/K 〈 1 and increases significantly the Rayleigh number for a neutral disturbance when α/K 〉 1. It appears that under practical conditions disturbances with α/K above unity are unstable, although ignorance of the values of parameters characterizing a fluidized bed hinders quantitative conclusions. © 1993, Cambridge University Press. All rights reserved.

Description: In this paper we discuss the possibility that concentric vortex rings, with associated circulation of opposite sign, can propagate steadily as a coherent pair. Inviscid flow considerations suggest that such a configuration, which we define as a vortex ring pair, may be possible. Numerical solutions of the Navier-Stokes equations for incompressible, laminar flow show that, although diffusion results in a continual redistribution of vorticity, a quasi-steadily propagating vortex ring pair could be attained in practice. Experiments are reported to test this idea by generating counter-rotating vortex ring pairs by impulsive fluid motion through an annular orifice. Depending on the normalized impulse and the orifice radius ratio, the vortex rings are observed either (i) to propagate together until diffusive effects or vortex ring instability destroys the coherent motion, or (ii) the inner ring propagates to some maximum axial distance where it reverses its direction and returns to the orifice wall, leaving the outer ring free to continue its forward motion unabated. Numerical simulation shows that the stable flow of the vortex ring trajectories can be reasonably well reproduced. The boundary separating motion (i) from (ii) and the normalized inner ring penetration distance are found over the range of impulse and radius ratio covered by the experiments. Other observed features of vortex ring motion including self-similar trajectories of the spiral core centres during vortex sheet roll-up and ring instability are also presented. © 1993, Cambridge University Press. All rights reserved.

Description: The effect of rapid mean compression on compressible turbulence at a range of turbulent Mach numbers is investigated. Rapid distortion theory (RDT) and direct numerical simulation results for the case of axial (one-dimensional) compression are used to illustrate the existence of two distinct rapid compression regimes. These regimes - the nearly solenoidal and the ‘pressure-released’ - are defined by a single parameter involving the timescales of the mean distortion, the turbulence, and the speed of sound. A general RDT formulation is developed and is proposed as a means of improving turbulence models for compressible flows. In contrast to the well-documented observation that ‘compressibility’ (measured, for example, by the turbulent Mach number) is often associated with a decrease in the growth rate of turbulent kinetic energy, we find that under rapid distortion compressibility can produce an amplification of the kinetic energy growth rate. We also find that as the compressibility increases, the magnitude of the pressure-dilation correlation increases, in absolute terms, but its relative importance decreases compared to the magnitude of the kinetic energy production. © 1993, Cambridge University Press. All rights reserved.

Description: We show that a recent publication by Liron & Barta (1992) concerning a single-layer boundary integral equation for the tractions is mathematically equivalent to Karrila & Kim’s (1989) Riesz method. In actual computational schemes, the second viewpoint is preferable since the integral operator has a spectral radius less than one and even large problems can be solved by fast iterative methods. © 1993, Cambridge University Press. All rights reserved.

Description: We consider steady, two-dimensional viscous flow of two fluids near a corner. The two fluids meet at the wedge vertex and are locally in contact with each other along a straight line emanating from the corner. The double wedge, treated in polar coordinates, admits separable solutions with bounded velocities at the corner. We seek local solutions which satisfy all local boundary conditions, as well as partial local solutions which satisfy all but the normal-stress boundary conditions. We find that local solutions exist for a wide range of total wedge angles and that a class of individual wedge angles and stress exponents is selected. Partial local solutions exist for all combinations of individual wedge angles and the stress exponents are determined as functions of these angles and the viscosity ratio. In both cases, Moffatt vortices can be found. Our aim in this work is to describe local two-fluid flow by determining for which wedge angles solutions exist, identifying singularities in the stress at the corner and identifying conditions under which Moffatt vortices can be present in the flow. Furthermore, for the single-wedge geometry, we identify for small capillary number non-uniformities present in solutions valid near the corner. © 1993, Cambridge University Press. All rights reserved.

Description: When an alternating electric field is applied to a colloid the particles oscillate at a velocity proportional to the applied field strength. The complex constant of proportionality is termed the dynamic mobility (O'Brien 1988). Although this quantity can now be determined from electroacoustic measurements in suspensions of arbitrary concentration (O'Brien 1990), the theory for interpreting these measurements in terms of the size and charge of the particles is limited to dilute suspensions. In this paper we derive an expression for the O(φ) correction to the dynamic mobility in a random suspension of uniform spheres with volume fraction φ. It is assumed that the particle radius is much greater than the double layer thickness but much smaller than the sound wavelength. The mobility is calculated using O'Brien's 1979 macroscopic boundary integral technique. This method ensures a correct mathematical formulation of the problem, and yields an absolutely convergent expression for the average particle velocity. The evaluation of this expression to O(φ) involves the determination of the velocities of an isolated pair of particles at various separations and frequencies of oscillation. These velocities are computed using the collocation technique and the O(φ) correction to the dynamic mobility is then obtained by numerically integrating over all particle separations. © 1993, Cambridge University Press. All rights reserved.

Description: The nonlinear interactions between a free surface and a shed vortex shear layer in the inviscid wake of a vertical surface-piercing plate are studied numerically using a mixed-Eulerian-Lagrangian method. For a plate with initial submergence d starting abruptly from rest to constant horizontal velocity U, the problem is governed by a single parameter, the Froude number Fn = U/(gd)½, where g is the gravitational acceleration. Depending on Fn, three classes of interaction dynamics (subcritical, transcritical and supercritical) are identified. For subcritical Fn (≲ 0.7), the free surface plunges on both the forward and lee sides of the plate before significant interactions with the vortex sheet occur. For transcritical and supercritical Fn, interactions between the free surface and the starting vortex result in a stretching of the vortex sheet which eventually rolls up into double-branched spirals as a result of Kelvin-Helmholtz instability. In the transcritical range (Fn ∼ 0.7-1.0), the effect of the free surface on the double-branched spirals remains weak, while for supercritical Fn (≳ 1.0), strong interactions lead to entrainment of the double-branched spiral into the free surface resulting in prominent surface features. © 1993, Cambridge University Press. All rights reserved.

Description: An integral method is developed to solve the two-dimensional Stokes problem with Neumann boundary conditions for multiply connected domains in which the inside hole area can shrink and disappear. The method is applied to simulate viscous sintering. In particular the sintering of glasses can be modelled as such, i.e. a viscous incompressible Newtonian volume flow driven solely by surface tension. A Boundary Element Method is applied to solve the integral equations of Stokes flow involved, and the time integration is carried out by a variable-step, variable-order Backward Differences Formulae method. The derived numerical algorithm is demonstrated for several arbitrarily shaped multiply connected sintering domains. In particular some cylindrical packings are considered. The latter simulations provide a justification for the use of ‘unit problems’ in the theory of sintering. © 1993, Cambridge University Press. All rights reserved.

Description: A variational formulation for three-dimensional waves in a continuously stratified shear flow is used to derive the equations governing a resonant triad of waves. It is argued that in general, critical layers are necessary for the existence of explosive resonant triads. © 1993, Cambridge University Press. All rights reserved.

Description: Amol’d’s second stability theorem is proved for arbitrary perturbations of the potential vorticity field 5q and the circulations) 8y. The formal stability condition is essentially the same as that for δγ ≡ 0, which is much easier to obtain. Similarly, the condition obtained assuming δq ≡ 0 (the overbar denoting a horizontal average) is found to be also valid for δq ≡ 0. It is argued that a Lyapunov functional that is extreme only on the sheet of constant Casimirs (and other integrals of motion) also proves stability for perturbations off the sheet, even though its second variation may not be sign definite for general perturbations. This conjecture is illustrated by means of a very simple mechanical problem: a point particle subject to the action of a central force. For the case of Phillips’ problem in a periodic channel, formal stability conditions on the isovortical sheet coincide with the criteria obtained from normal modes analysis. © 1993, Cambridge University Press. All rights reserved.

Description: It is known that the response of a cylindrical acoustic resonator to excitation by an oscillating piston can contain shock waves if the detuning is sufficiently small. However, the response of a spherical annular resonator is continuous, with an amplitude that depends on the detuning in the same way as does a Duffing equation. This paper discusses the response in resonators that deviate from being cylindrical and shows that, in general, the detuning range in which shocks are possible decreases as the geometrical imperfection increases. © 1993, Cambridge University Press. All rights reserved.

Description: The motion of an elliptical cylindrical particle immersed in an incompressible Newtonian fluid in a narrow channel is examined numerically in the zero-Reynolds-number limit. It is assumed that no external forces or torques act on the elliptical cylinder, and the effects of inertia forces on the motion of the fluid and the particle are neglected. The Stokes equations are solved by a finite-element method for various positions and orientations of the cylinder, yielding the instantaneous velocities of the particle that satisfy the conditions of zero force and zero torque on the particle. Using the computed longitudinal, lateral, and angular velocities of the particle, the evolution of the particle’s position and orientation is determined for various initial configurations. An elliptical cylinder is found to either tumble or oscillate in rotation, depending on the particle-channel size ratio, the axis ratio of the elliptical cylinder, and the initial conditions. In the first case, the particle rotates continuously in one direction that is well approximated by Jeffery’s solution for an elliptical cylinder in unbounded shear flow with a so-called equivalent axis ratio; in the second case, the particle changes its direction of rotation during part of each period. In both cases, the particle translates with a periodically varying longitudinal velocity, accompanied by a considerable side drift due to the walls. The oscillatory motion is more likely to occur when the particle-channel size ratio or axis ratio is increased. The tumbling motion is inhibited for elliptic cylinders whose size ratios are larger than threshold values that depend on the axis ratio. © 1993, Cambridge University Press. All rights reserved.

Description: Experiments are presented for three turbulent boundary layers generated by laterally converging, laterally diverging and parallel flow on a flat plate. A converging potential flow field outside the boundary layer was generated by superposing a parallel flow in the -direction, a row of equally spaced line sources in the wall-normal (y) direction and an analogous row of sinks in the transversal (z) direction. This arrangement resulted in a velocity that was constant far upstream, far downstream and along the x-axis. The convergence — ∂W/∂z has its maximum in the plane of the source and sink rows. This flow field was realized with the test section shown in figure 1, based on streamlines intersecting a rectangular cross-section far upstream. The diverging flow was generated by reversing the flow direction through the test section. The tests were conducted at about 42 m/s leading to a unit Reynolds number of 2.5 x 106/m and to a Reynolds number based on the momentum thickness of 4000 to 4700 at the inlet of the test sections, increasing up to 25000 at the outlet. In all three cases the velocity distribution near the wall agreed very well with the logarithmic law of the wall. The wake contribution in the outer layer was considerably increased by convergence and decreased by divergence. The Reynolds stresses, measured with crossed hot-wire probes, and the wall shear stress, measured with a floating-element balance, were generally increased by divergence and decreased by convergence and the same holds true for the mixing length and the turbulent viscosity. A finite-difference boundary-layer code using a simple turbulence model was used to predict the experimental results. The comparison showed good agreement for the two-dimensional flow, reasonable agreement for the diverging flow and poor agreement for the converging one. Use of the experimentally determined turbulent viscosity as input into the computation did not systematically improve the agreement but excellent agreement was found if it was combined with anisotropy of the turbulent viscosity. It was much more difficult to predict the converging flow as small errors in the crossflow had a large effect on the flow in the plane of symmetry (z = 0). © 1993, Cambridge University Press. All rights reserved.

Description: The nonlinear dynamical behaviour of a conducting drop in a time-periodic electric field is studied. Taylor’s (1964) theory on the equilibrium shape is extended to derive a dynamical equation in the form of an ordinary differential equation for a conducting drop in an arbitrary time-dependent, uniform electric field based on a spheroidal approximation for the drop shape and the weak viscosity effect. The dynamics is then investigated via the classical two-timing analysis and the Poincare map analysis of the resulting dynamical equation. The analysis reveals that in the neighbourhood of a stable steady solution, an O(e) time-dependent change of drop shape can be obtained from an 0(e) resonant forcing. It is also shown that the probability of drop breakup via chaotic oscillation can be maximized by choosing an optimal frequency for a fixed forcing amplitude. As a preliminary analysis, the effect of weak viscosity on the oscillation frequency modification in a steady electric field is also studied by using the domain perturbation technique. Differently from other methods based on the theory of viscous dissipation, the viscous pressure correction is directly obtained from a consideration of the perturbed velocity field due to weak viscosity. © 1993, Cambridge University Press. All rights reserved.

Description: Experiments are performed on axisymmetric spreading of viscous drops on glass plates. Two liquids are investigated: silicone oil (M-100), which spreads to ‘infinity’, and paraffin oil, which spreads to a finite-radius steady state. The experiments with silicone oil partly recover the behaviour of previous workers’ data; those experiments with paraffin oil provide new data. It is found that gravitational forces dominate at long enough times while at shorter times capillary forces dominate. When the plate is heated or cooled with respect to the ambient gas, thermocapillary forces generate flows that alter the spreading dynamics. Heating (cooling) the plate is found to retard (augment) the spreading. Moreover, in case of partial wetting, the drop radius finally approached is smaller (larger) for a heated (cooled) plate. These data are all new. All these observations are in good quantitative agreement with the related model predictions of Ehrhard & Davis (1991). A breakdown of the axisymmetric character of the flow is observed only for very long times and/or very thin liquid layers. © 1993, Cambridge University Press. All rights reserved.

Description: We discuss a series of numerical experiments on the dispersion of neutrally buoyant particles in two-dimensional turbulent flows. The topology of two-dimensional turbulence is parametrized in terms of the relative dominance of deformation or rotation; this leads to a segmentation of the turbulent field into hyperbolic and elliptic domains. We show that some of the characteristic structural domains of two-dimensional turbulent flows, namely coherent structures and circulation cells, generate particle traps and peculiar accelerations which induce several complex properties of the particle dispersion processes at intermediate times. In general, passive particles are progressively pushed from the coherent structures and tend to concentrate in highly hyperbolic regions in the proximity of the isolines of zero vorticity. For large dispersion times, the background turbulent field is a privileged domain of particle richness; there is however a permanent particle exchange between the background field and the energetic circulation cells which surround the coherent structures. At intermediate times, an anomalous dispersion regime may appear, depending upon the relative weight of the different topological domains active in two-dimensional turbulence. The use of appropriate conditional averages allows the basic topology of two-dimensional turbulence to be characterized from a Lagrangian point of view. In particular, an intermediate P anomalous dispersion law is shown to be associated with the action of hyperbolic regions where deformation dominates rotation; the motion of the advected particles in strongly elliptic regions where rotation dominates over deformation is shown to be associated with a ti dispersion law. Because neutral particles concentrate on average in hyperbolic regions, the P dispersion law is quite robust and it can be observed under very general circumstances. © 1993, Cambridge University Press. All rights reserved.

Description: We consider a horizontal three-dimensional saturated porous layer, confined in an upright cubic box, heated from below and cooled from above. In the absence of a controller, the fluid maintains a no-motion state for subcritical Rayleigh numbers R 〈 Rc, where Rcdepends on the box’s aspect ratio. Once this critical number is exceeded, fluid motion ensues. We demonstrate that, with the use of feedback control strategies which suppress flow instabilities, one can maintain a stable no-motion state for Rayleigh numbers far exceeding the classical critical one for the onset of convection. To preserve the equilibrium no-motion state of the classical problem, the controller alters the system’s dynamics so as to stabilize an otherwise non-stable state. © 1993, Cambridge University Press. All rights reserved.

Description: Several aspects of the growth and departure of bubbles from a submerged needle are considered. A simple model shows the existence of two different growth regimes according to whether the gas flow rate into the bubble is smaller or greater than a critical value. These conclusions are refined by means of a boundary-integral potential-flow calculation that gives results in remarkable agreement with experiment. It is shown that bubbles growing in a liquid flowing parallel to the needle may detach with a considerably smaller radius than in a quiescent liquid. The study also demonstrates the critical role played by the gas flow resistance in the needle. A considerable control on the rate and size of bubble production can be achieved by a careful consideration of this parameter. The effect is particularly noticeable in the case of small bubbles, which are the most difficult ones to produce in practice. © 1993, Cambridge University Press. All rights reserved.

Description: Direct numerical simulations of fully developed pressure-driven turbulent flow in a rotating channel have been performed. The unsteady Navier-Stokes equations were written for flow in a constantly rotating frame of reference and solved numerically by means of a finite-difference technique on a 128 x 128 x 128 computational mesh. The Reynolds number, based on the bulk mean velocity Um and the channel half-width h, was about 2900, while the rotation number Ro = 2 Ω/h/Um varied from 0 to 0.5. Without system rotation, results of the simulation were in good agreement with the accurate reference simulation of Kim, Moin & Moser (1987) and available experimental data. The simulated flow fields subject to rotation revealed fascinating effects exerted by the Coriolis force on channel flow turbulence. With weak rotation (Ro = 0.01) the turbulence statistics across the channel varied only slightly compared with the nonrotating case, and opposite effects were observed near the pressure and suction sides of the channel. With increasing rotation the augmentation and damping of the turbulence along the pressure and suction sides, respectively, became more significant, resulting in highly asymmetric profiles of mean velocity and turbulent Reynolds stresses. In accordance with the experimental observations of Johnston, Halleen & Lezius (1972), the mean velocity profile exhibited an appreciable region with slope 2Q. At Ro = 0.50 the Reynolds stresses vanished in the vicinity of the stabilized side, and the nearly complete suppression of the turbulent agitation was confirmed by marker particle trackings and two-point velocity correlations. Rotational-induced Taylor-Görtler-like counter-rotating streamwise vortices have been identified, and the simulations suggest that the vortices are shifted slightly towards the pressure side with increasing rotation rates, and the number of vortex pairs therefore tend to increase with Ro. © 1993, Cambridge University Press. All rights reserved.

Description: A direct numerical simulation of a fully developed, low-Reynolds-number turbulent flow in a square duct is presented. The numerical scheme employs a time-splitting method to integrate the three-dimensional, incompressible Navier-Stokes equations using spectral/high-order finite-difference discretization on a staggered mesh; the nonlinear terms are represented by fifth-order upwind-biased finite differences. The unsteady flow field was simulated at a Reynolds number of 600 based on the mean friction velocity and the duct width, using 96 × 101 × 101 grid points. Turbulence statistics from the fully developed turbulent field are compared with existing experimental and numerical square duct data, providing good qualitative agreement. Results from the present study furnish the details of the corner effects and near-wall effects in this complex turbulent flow field; also included is a detailed description of the terms in the Reynolds-averaged streamwise momentum and vorticity equations. Mechanisms responsible for the generation of the stress-driven secondary flow are studied by quadrant analysis and by analysing the instantaneous turbulence structures. It is demonstrated that the mean secondary flow pattern, the distorted isotachs and the anisotropic Reynolds stress distribution can be explained by the preferred location of an ejection structure near the corner and the interaction between bursts from the two intersecting walls. Corner effects are also manifested in the behaviour of the pressure-strain and velocity-pressure gradient correlations. © 1993, Cambridge University Press. All rights reserved.

Description: Boiling and natural-convection processes in a horizontal, fluid-saturated porous layer are investigated. The layer is heated uniformly from below and is cooled from above. Volumetric cooling is also allowed. The thermodynamic structure consists of a liquid region overlying a two-phase region. Numerical techniques are used to solve the transient, two-dimensional equations in the liquid and two-phase regions, and at the phase-change interface. A parametric study is carried out in terms of the liquid-phase Rayleigh number (Ra) and the non-dimensional bottom heat flux (Qb). Three solution regimes are observed: conduction-dominated at low Ra, convection-dominated at intermediate Ra, and oscillatory convection at high Ra. In the convection-dominated regime, transitions to multiple cell patterns are observed as Qbis increased. Oscillatory convection appears to be triggered by asymmetric disturbances in the system. The effects of initial conditions and the stability of the solutions to perturbations are also investigated. The heat transfer correlations and qualitative flow patterns are in agreement with experiments. © 1993, Cambridge University Press. All rights reserved.

Description: It is shown that the exponential growth rate of the fast kinematic dynamo instability can be related to the Lagrangian stretching properties of the underlying chaotic flow. In particular, a formula is obtained relating the growth rate to the finite time Lyapunov numbers of the flow and the cancellation exponent K. (The latter quantity characterizes the extremely singular nature of the magnetic field with respect to fine-scale spatial oscillation in orientation.) The growth rate formula is illustrated and tested on two examples: an analytically soluble model, and a numerically solved spatially smooth temporally periodic flow. © 1993, Cambridge University Press. All rights reserved.

Description: The propagation of weakly nonlinear acoustic waves in a non-uniform medium is treated. It is assumed that the waves are one-dimensional. Non-uniformities arising from variable cross-section and stratification are included. The effect of nonuniformities on unidirectional waves on an infinite interval and resonant waves on a finite interval is discussed for a near-uniform reference state (geometrical acoustics limit) and for stronger non-uniformities in the finite-interval case. Nonlinearities are taken into account up to quadratic and, wherever necessary, cubic order in the wave amplitude. Unidirectional waves in the geometrical acoustics limit can formally be reduced to the behaviour in a uniform system described by a kinematic wave equation with constant coefficients. For illustration acceleration waves in a weakly non-uniform medium are treated. The resonance case in the geometrical acoustics limit is closely related to resonance in a uniform system so that the methods developed for that situation require only slight modification. For larger influence of non-uniformity the geometrical acoustics limit does not apply and the resonance problem may lead to a Duffing oscillator type of behaviour. © 1993, Cambridge University Press. All rights reserved.

Description: The geographical distribution and depth zonation of Eusirus perdentatus Chevreux, 1912 in the eastern Weddell Sea and adjacent Lazarev Sea (Antarctica) is described. A total of 963 individuals of this carnivorous predator caught during six successive cruises at 71 stations between 176 and 799 metres water depths were used. Individuals of E. perdentatus have been kept alive for nearly five months in the laboratory. During this period females released 53 juveniles in April. Oocytes and embryos of females have been counted and measured. The duration of embryonic development in E. perdentatus was estimated at c. 12 months, using an empirical relationship based on the mean diameter of 4390 fertilized eggs. There is strong evidence that this species is semelparous with hatching of juveniles at the end of austral summer. Two methods, herein termed as Year Class Model and Moult Class Model, have been applied in order to describe the growth of females by means of cumulative length-frequency data.

Description: Maritime Antarctic freshwater lakes and their catchments are inherently simple systems in an environment which is characterized by strong seasonality. Such lakes offer excellent opportunities to study the interaction of water chemistry and plant communities. The response of diatom species to environmental gradients was assessed by constructing a diatom and water chemistry dataset from 59 lakes at two locations (Livingston Island, South Shetland Islands and Signy Island, South Orkney Islands). Results indicate that diatom species abundance is predominately related to nutrient and salinity gradients. The dataset will be used to create transfer functions which can be applied to sediment core diatom assemblages to reconstruct historical patterns of lake chemistry.

Description: Temperature records in the Antarctic Peninsula have shown a climatic warming of 1.5°C over the past 30 years and a number of ice shelves have retreated. The most dramatic retreat has been that of Wordie Ice Shelf which has undergone a catastrophic disintegration since the 1960s. Understanding the cause and mechanism of the break-up may provide important clues to the fate of ice shelves farther south which, it has been suggested, help to stabilize the West Antarctic Ice Sheet. The break-up of Wordie Ice Shelf has been analysed using Landsat and SPOT imagery. These observations show that the relative contribution of the various input glaciers to the grounding line flux has not altered during the break-up. This means that the effect of the rapid and almost complete removal of the ice shelf has not been transmitted upstream and is not causing a rapid increase in velocities on the input glaciers. The volume of grounded ice in the catchment of Wordie Ice Shelf will thus, be largely unaffected by the break-up and there will be no significant contribution to sea level change. Since other ice shelves around the Antarctic Peninsula are also fed by relatively steep mountain glaciers the effect of the loss of the ice shelves on sea level would be likely to be similarly small.

Description: A varied suite of sapphirine-bearing and quartz-undersaturated granulites, the Taynaya Paragneiss, occur as boudins and enclaves within the c. 2500 Ma old felsic orthogneisses of northern Vestfold Hills. Highly magnesian varieties with XMg (=100x(Mg/(Mg+Fe)) near 95 preserve the assemblage sapphirine + enstatite + spinel, whereas sapphirine + cordierite + sillimanite + corundum occurs in aluminous and feldspathic types with XMg near 90. Phase equilibria and relative thermometry based on Al2O3 solubility in enstatite indicate equilibration of these assemblages at c. 830–880°C and 0.35–0.85 GPa. There is no evidence for the extreme temperatures (1000–1100°C) previously proposed for early metamorphism in the Vestfold Hills, and no indication in the metamorphic assemblages of isobaric cooling prior to 2500 Ma. Two types of metasomatism have altered the bulk rock compositions near boudin and enclave margins. Cordierite rinds locally formed on corundum-sillimanite granulites reflect interaction with magmatic precursors to the enclosing felsic gneisses, as supported by the isotopic and chemical compositions of cordierite channel volatiles. More extensive metasomatism producing schistose phlogopite + sapphirine rinds on all boudins involved infiltration of a LILE-enriched fluid which introduced K2O, H2O, Fe, Rb, Ba and minor Sr along the boudin margins. Whole rock geochemistry of Taynaya Paragneiss unaffected by this metasomatism is consistent with their derivation from evaporitic mudstones, and implies the existence of a basement older than the dominant 2500 Ma orthogneisses.