ALBERT

Description: In this paper we examine the linear stability of an annular film surrounding a dielectric-fluid core in a tube in the presence of double layers of charges at the film core and at the film—tube interfaces, when the fluid—fluid interface is of low tension. In the absence of electrostatic forces, the surface tension force arising from the circumferential curvature destabilizes, and that from the axial curvature stabilizes the system. The competition is such that waves larger than the unperturbed interface circumference are unstable and those shorter are stable. For charged layers in the film, two cases are examined (i) double-layer repulsion where the volume charge density is everywhere of the same sign and (ii) double-layer attraction where the diffusive layers next to the film interfaces are of opposite signs. In the first case, double-layer repulsion and surface tension lowering stabilize the destabilizing action of the circumferential component of the surface tension force, and a window of stability can exist. In the case of double layers of opposite signs, double-layer attraction destabilizes the system, and growth rates larger than those caused by pure capillarity can arise. Finally, for the case of a core bounded by an infinite electrolyte, surface tension lowering stabilizes the destabilizing action of the circumferential component of the surface tension force and destabilizes the longitudinal one, although the magnitudes of these effects may differ. As a result the thread can become unstable to waves shorter than the interface circumference. © 1991, Cambridge University Press. All rights reserved.

Description: The linear stability of filaments or strips of ‘potential’ vorticity in a background shear flow is investigated for a class of two-dimensional, inviscid, non-divergent models having a linear inversion relation between stream function and potential vorticity. In general, the potential vorticity is not simply the Laplacian of the stream function — the case which has received the greatest attention historically. More general inversion relationships between stream function and potential vorticity are geophysically motivated and give an impression of how certain classic results, such as the stability of strips of vorticity, hold under more general circumstances. In all models, a strip of potential vorticity is unstable in the absence of a background shear flow. Imposing a shear flow that reverses the total shear across the strip, however, brings about stability, independent of the Green-function inversion operator that links the stream function to the potential vorticity. But, if the Green— function inversion operator has a sufficiently short interaction range, the strip can also be stabilized by shear having the same sense as the shear of the strip. Such stabilization by ‘co-operative’ shear does not occur when the inversion operator is the inverse Laplacian. Nonlinear calculations presented show that there is only slight disruption to the strip for substantially less adverse shear than necessary for linear stability, while for co-operative shear, there is major disruption to the strip. It is significant that the potential vorticity of the imposed flow necessary to create shear of a given value increases dramatically as the interaction range of the inversion operator decreases, making shear stabilization increasingly less likely. This implies an increased propensity for filaments to ‘roll-up’ into small vortices as the interaction range decreases, a finding consistent with many numerical calculations performed using the quasi-geostrophic model. © 1991, Cambridge University Press. All rights reserved.

Description: Techniques of bifurcation theory are used to study the porous-medium analogue of the classical Rayleigh-Benard problem, Lapwood convection in a two-dimensional saturated porous cavity heated from below. The focus of the study concerns the destabilization, through symmetry-preserving Hopf bifurcations, of the various stable convective flow patterns that can form in a rectangular cavity. We show how the limits of stability of steady convection in a porous medium can be determined by bifurcation techniques that locate Hopf bifurcations, and we predict a surprisingly complex evolution of the Hopf bifurcation along the unicellular branch as the aspect ratio varies. The continuation methods that we adopt reveal interactions of Hopf bifurcations with limit points that signal complicated dynamical behaviour for certain container sizes. The study demonstrates the role of Hopf bifurcation in destabilizing completely the unicellular flow at aspect ratios greater than 2.691. A simple relationship between symmetry-preserving Hopf bifurcations from the alternative steady flows is also derived, and used to define upper limits on the stability of the alternative steady flows and the thresholds for oscillatory convection as a function of aspect ratio. © 1991, Cambridge University Press. All rights reserved.

Description: Grosch & Salwen (1978) discuss the continuous-spectrum contribution in both temporal and spatial stability problems that are governed by the linear Orr–Sommerfeld equation. Their computed temporal continuum eigenfunctions for the Blasius boundary layer and for a laminar jet profile show surprising differences. This note provides an improved physical understanding of the results through a simple model, and shows that these differences are more apparent than real. © 1991, Cambridge University Press. All rights reserved.

Description: In experiments to measure the surface energy of a magnetically levitated molten metal droplet by observation of its oscillation frequencies, Rayleigh’s equation is usually used. This assumes that the equilibrium shape is a sphere, and the surface restoring force is due only to surface tension. This work investigates how the vibrations of a non-rotating liquid droplet are affected by the asphericity and additional restoring forces that the levitating field introduces. The calculations show that the expected single frequency of the fundamental mode is split into either three, when there is an axis of rotational symmetry, or five unequally spaced bands. Frequencies, on average, are higher than those of an unconstrained droplet; the surface tension appears to be increased over its normal value. This requires a small correction to be made in all analyses of surface energy. A frequency sum rule is derived from a simplified model of the magnetic field which allows the corresponding Rayleigh frequency to be evaluated from the observed frequencies of the fundamental and translational modes. A more detailed analysis shows a similar correction but one that is also sensitive to the position of the droplet in the field. © 1991, Cambridge University Press. All rights reserved.

Description: SUMMARYComponents of the N cycle were studied at Hurley, UK, in 1985–87. In grass-clover (Lolium perenne and Trifolium repens) swards, grazed at three intensities, low total inputs of N were associated with low outputs and losses of N. Nevertheless, the flows (intake and excretion) of N through animals were substantial and gave rise, at the higher intensities of grazing, to an acceptably high agricultural output per hectare. This was considered evidence of a fast and efficient recycling of N between plants, animals and soil. The release of N to the environment (as nitrogenous gases and nitrate) was substantially less from the grass–clover swards than from a grass sward fertilized with 420 kg N/ha, and this was at the expense of only 20% loss in production. The mechanisms which might account for the high efficiency of utilization and recycling of N in grass–clover swards are discussed in the context of the balance of the supply of C and N to plant and soil biomasses under grazing. The results confirm that optimizing agricultural output in grass–clover swards has little adverse effect on the environment, but the importance to this end of sustaining a large proportion of N-deficient grass in grass-clover swards is emphasized.

Description: SUMMARYA total of 52 individually penned pigs (mean initial and final live weights (LW) of 31·7 and 81·5 kg, respectively) was used in a randomized block experiment at the Agricultural Research Institute of Northern Ireland, Hillsborough, in 1984. The treatments were based on dietary regimes which comprised a control diet in which meal was offered at 86 g dry matter/k W0·75 per day or diets in which silage effluent was substituted for part of the allowance of meal of the control diet at 125, 175, 225 and 275 g/kg on a dry matter basis to give 0·125, 0·175, 0·225 and 0·275 effluent diets. There was also a negative control diet in which meal was offered at 850 g/kg of the rate of feeding of the control diet. Each of these dietary treatments included water available ad libitum. In a further treatment, effluent was substituted for 225 g/kg of the meal allowance of the control treatment on a dry matter basis, but additional water was not offered. The treatment based on the 0·275 effluent diet was omitted from the first four replicates.Some refusals of diet occurred at the higher rates of effluent feeding, with mean refusals of 7, 11 and 21 g/kg, respectively, for the 0·175, 0·225 and 0·275 effluent diets on a dry matter basis. Significant linear and quadratic contrasts (P

Description: SUMMARYThe intake of temperate forages by herbivores can be determined using the C33 alkane naturally present in the forage and dosed C32 alkane. To determine whether the technique can be used with tropical forages, the concentrations of C33 alkane were determined in seven species.The leaves of Brachiaria decumbens, Digitaria decumbens, Pennisetum glaucum and Stylosanthes scabra contained sufficient C33 alkane (〉 50 mg/kg DM) for the estimation of dry matter intake. Low concentrations of C33 alkane were found in the leaves of Setaria sphacelata, Sorghum sp. and Leucaena leucocephala. Setaria sphacelata was found to contain high concentrations of C27 alkane and therefore intake could be estimated by using C28 as the dosed alkane although the intake would possibly then be underestimated by c. 8%. Leaves of Sorghum sp. contained 〉 50 mg C31 alkane/kg DM and, with C32 as the dosed alkane, intake would possibly be underestimated by c. 5%. Leucaena leucocephala contained insufficient alkanes to estimate forage intake. The concentration of n-alkanes in Pennisetum glaucum and Sorghum sp. leaves decreased with age.It was concluded that some tropical forages contain insufficient C33 alkane for the measurement of intake by the double alkane technique. In these species, a shorter chain length alkane can sometimes be used, but only with a reduction in accuracy in estimating intake.

Description: SUMMARYFeeding studies were conducted in 1986 with penned sheep and cattle fed legume (C3) and grass (C4) hays mixed in different proportions. Large fluctuations in diet δ13C within or between days were associated with much smaller fluctuations in the δ13C of faeces. When the overall daily legume percentage in the diet was held constant, there was little variation in the δ13C of faeces irrespective of when during the day the legume, relative to the grass, was eaten.Cattle set stocked on Stylosanthes/grass pastures in Queensland showed only minor variations in faecal δ13C within or between days in the short term. All animals within a group showed similar trends when the diet was changing. The changes in the δ13C of faeces from day to day reflected small but real changes in diet selection.It was concluded that the δ13C of a single faecal sample reliably reflects the integrated diet over the previous 3–4 days of a free-grazing ruminant so that the frequency and pattern of faecal sampling for estimating diet composition may be determined by considerations other than reliability

Description: SUMMARYAgronomic research was carried out on upland rice in Toledo District, Belize, in 1979–86, with the objective of improving returns from the traditional system of shifting cultivation and of examining the potential for reducing the demand for new land by cropping for more than one season. The rice was grown in the wet season on land cleared from forest by slash and burn, using a traditional planting technique, placing seed in holes made with a pointed stick.In a series of fertilizer experiments, in which rice was grown for up to 7 consecutive years, urea and triple superphosphate both increased grain yield, whereas muriate of potash had no effect. Mean yield in the fifth year was 85 % of that in the first. Urea and triple superphosphate both increased the number of full grains/panicle.A slash and mulch weed control treatment before sowing resulted in low yields of grain in the third year; yield after the use of propanil and 2,4-D (after sowing) was, on average, 77% of the yield after hand weeding; omitting weed control after sowing led to very low yields, particularly in the second and third years.In the varieties Bluebonnet and C22, grain yield was increased by reducing the distance between stations, particularly where 6, rather than 15, seeds were sown per station. Reducing the distance between stations increased the leaf area index during crop development and the number of panicles/m2, which more than counterbalanced reductions in the number and area of leaf blades per plant, the number of panicles per plant and the number of full grains per panicle. However, the yield/kg of seed sown and per hour spent planting was greatest at the widest spacing. C22 produced more panicles per plant and a greater weight of grain per plant and per hectare than Bluebonnet.It was concluded that returns from the traditional system can be improved and the demand for new land reduced.