ALBERT

Description: Disturbance interactions in wave triads and multiwave systems of various configurations are investigated to reveal the mechanism of laminar-turbulent transition in Blasius and pressure-gradient boundary layers. The averaging method of weakly nonlinear instability theory in quasi-parallel flows is applied. Tollmien-Schlichting-wave resonant interaction is shown to be the only leading mechanism of subharmonic (S)-type transition. The mechanism universally dominates in boundary layers excited by sufficiently small initial disturbances. The role of any other mode is inefficient. Weakly nonlinear models are concluded not to explain the K-type transition scenario. The results of the study are employed to interpret physical and numerical experimental data. © 1993, Cambridge University Press. All rights reserved.

Description: The propagation of guided internal waves on non-uniform large-scale flows of arbitrary geometry is studied within the framework of linear inviscid theory in the WKB-approximation. Our study is based on a set of Hamiltonian ray equations, with the Hamiltonian being determined from the Taylor-Goldstein boundary-value problem for a stratified shear flow. Attention is focused on the fundamental fact that the generic smooth non-uniformities of the large-scale flow result in specific singularities of the Hamiltonian. Interpreting wave packets as particles with momenta equal to their wave vectors moving in a certain force field, one can consider these singularities as infinitely deep potential holes acting quite similarly to the ‘black holes’ of astrophysics. It is shown that the particles fall for infinitely long time, each into its own ‘black hole’. In terms of a particular wave packet this falling implies infinite growth with time of the wavenumber and the amplitude, as well as wave motion focusing at a certain depth. For internal-wave-field dynamics this provides a robust mechanism of a very specific conservative and moreover Hamiltonian irreversibility. This phenomenon was previously studied for the simplest model ot the flow nonuniformity, parallel shear flow (Badulin, Shrira & Tsimring 1985), where the term ‘trapping’ for it was introduced and the basic features were established. In the present paper we study the case of arbitrary flow geometry. Our main conclusion is that although the wave dynamics in the general case is incomparably more complicated, the phenomenon persists and retains its most fundamental features. Qualitatively new features appear as well, namely, the possibility of three-dimensional wave focusing and of ‘non-dispersive’ focusing. In terms of the particle analogy, the latter means that a certain group of particles fall into the same hole. These results indicate a robust tendency of the wave field towards an irreversible transformation into small spatial scales, due to the presence of large-scale flows and towards considerable wave energy concentration in narrow spatial zones. © 1993, Cambridge University Press

Description: The dependence of the parameters of capillary-gravity ripples on the characteristics of the steep surface waves (in the range 4–20 cm) that excite them is found. For steep 4–6 cm waves calculations are performed on the basis of the improved first Stokes method. Qualitative coincidence of the theoretical results with the experimental data is shown. For 7–20 cm waves the results are obtained by the multiple-scale method where the large-scale motion and the driving force for the ripple are found by the improved first Stokes method. Qualitative agreement between theory and experiment in this wavelength range is achieved. © 1991, Cambridge University Press. All rights reserved.

Description: The stability to linearized two-dimensional disturbances of plane Poiseuille flow of a Bingham fluid is considered. Bingham fluids exhibit a yield stress in addition to a plastic viscosity and this description is typically applied to drilling muds. A non-zero yield stress results in an additional parameter, a Bingham number, and it is found that the minimum Reynolds number for linear instability increases almost linearly with increasing Bingham number. © 1994, Cambridge University Press. All rights reserved.

Description: The paper is devoted to the theoretical and experimental investigation of the ring (toroidal) shock wave near the axis of symmetry. The theoretical approach is based upon the Chester–Chisnell–Whit man method. The experimental toroidal shock wave is generated by a novel inducer and visualized by the shadow technique. Attention is paid to the manner of reflection of the shock wave from the axis of symmetry. This reflection appears to be irregular even at small distances from the centre of the ring. This phenomenon is due to the cumulative acceleration of the converging axisymmetric shock front near the axis. The acceleration results in an increase in the incidence angle up to that characteristic of Mach (irregular) reflection. © 1991, Cambridge University Press. All rights reserved.

Description: When a horizontal force is applied locally to some volume of a viscous density-stratified fluid, flows with high concentration of vertically oriented vorticity (vortex dipoles) are generated. The processes of generation and evolution with time of theseunsteady flows in a stratified fluid are studied. A convenient way to produce and studythese flows in the laboratory is to use a submerged horizontal jet as a 'point’ source of momentum. The main governing parameter (the 'force’) is easily controlled in this case. Two regimes were studied starting jets with dipolar vortex fronts (the force acts continuously) and impulsive vortex dipoles (the force actsfor a short period of time). A conductivity microprobe, aluminium powder, shadowgraph, thymol-blue and other techniques have been used to measure the velocity and density distributions in the flows. It is found that in both regimes the flows are self-similar: thelengthscale of the flows increases with time as ft for starting jets and as t1/3 for vortex dipoles. Detailed information about the generation mechanism, kinematics and dynamicsof the flows is obtained. On the basis of similarity principles a theoretical explanation of the experimental results is given. The theory is in good agreement with the results obtained. © 1991, Cambridge University Press. All rights reserved.

Description: Late Archaean granitic rocks from the southern Yilgarn Craton of Western Australia have a close temporal relationship to the basaltic and komatiitic volcanism which occurs within spatially associated greenstone belts. Greenstone volcanism apparently began ∼2715 Ma ago, whereas voluminous felsic magmatism (both extrusive and intrusive) began about 2690 Ma ago. A brief but voluminous episode of crust-derived magmatism ∼2690-2685 Ma ago resulted in the emplacement of a diverse assemblage of plutons having granodioritic, monzogranitic and tonalitic compositions. This early felsic episode was followed immediately by the emplacement of mafic sills, and, after a further time delay, by a second episode of voluminous crust-derived magmatism dominated by monzogranite but containing plutons covering a wide compositional range, including diorite, granodiorite and tonalite. The products of this 2665–2660 Ma magmatic episode now form a significant fraction of the exposed southern Yilgarn Craton. Later magmatism, which continued to at least 2600 Ma ago, appears largely restricted to rocks having unusually fractionated compositions.The magmatic sequence basalt-voluminous crust-derived magmatism-later diverse magmatism, is interpreted in terms of a dynamically-based model for the ascent of the head of a new mantle plume. In this model basalts and komatiites are derived by decompression melting of rising plume material, and the crust-derived magmas result after conductive transport of heat from the top of the plume head into overlying continental crust. This type of magmatic evolution, the fundamentally bimodal nature of the magmatism, the presence of high-Mg volcanics (komatiites), and the areal extent of the late Archaean magmatic event, are all suggested to be characteristic of crustal reworking above mantle plumes rather than resulting from other processes, such as those related to subduction.

Description: This study deals with the first 2 years of grazing trials on improved pastures of tropical grasses and legumes at Ibadan in Nigeria. The mixtures consisted of (i) Cynodon plectostachyus and Centrosema pubescens (legume), (ii) (i)+Chloris gayana and Digitaria decumbens, (iii) (ii) +Stylosanthes gracilis (legume). The mixtures were sown and planted in four blocks, the treatments being repeated four times, and were grazed rotationally by N'Dama cattle. During the first year's trial a uniform stocking rate of 1·7 steers/acre was adopted and a recovery period of 11 weeks was allowed. In the second year's trial the stocking rate was increased to 2·7 beasts/acre by the addition of nine yearling heifers and the recovery period was reduced to 5 weeks. Herbage yield and intake were measured by the clipping technique.

Description: SUMMARYGroups of Awassi lambs, aged 7–13 months at start of the trial, were fed for 100 days on four different rations. A ‘control’ ration contained ground barley: the others contained different proportions of dried sugar-beet pulp, and no barley.Male lambs were initially heavier, and gave better weight gains than the females. Groups on beet pulp grew better than the ‘control’. The most economical weight gains were obtained in the group in which beet pulp constituted 40 % of the ‘concentrate’ portion of the diet.

Description: Perim Island is an eroded fragment of the southwest flank of a late Miocene (10.5 ± 1.0 Ma) volcano whose centre lay on the southwesternmost tip of Arabia. The volcano is the westernmost of the E–W line of six central vent volcanoes (the Aden Line) that extends 200 km along the south coast of Arabia from Perim to Aden. Major oxide and trace element abundances are given for 35 Perim specimens and these show that the volcano has within-plate trace element characteristics and consists of a petrographically and geochemically simple suite of alumina-poor olivine basalts, andesites, and transitional andesite–trachyandesites. Six specimens, however, are markedly enriched in Al2O3 and CaO, and contain abundant (20–30 mode %) highly calcic (An77–83) plagioclase phenocrysts. Geochemical modelling suggests that the main Perim volcanic sequence was produced by the fractional crystallization (o1 + cpx + Ti-mt + plag) of a silica saturated (SiO2 c. 45%) basic melt. The high A1, high Ca, magmas appear to be mixing products of plagioclase-enriched basic magmas with more evolved melts. Perim is the oldest volcano of the Aden line, which becomes increasingly younger and alkalic eastward. It is suggested that the volcanism is related to an eastwards-propagating rift produced before the most recent stage of sea-floor spreading in the Gulf of Aden (4.5 Ma–present).