ALBERT

Description: Flapping wings often feature a leading-edge vortex (LEV) that is thought to enhance the lift generated by the wing. Here the lift on a wing featuring a leading-edge vortex is considered by performing experiments on a translating flat-plate aerofoil that is accelerated from rest in a water towing tank at a fixed angle of attack of 15°. The unsteady flow is investigated with dye flow visualization, particle image velocimetry (PIV) and force measurements. Leading-and trailing-edge vortex circulation and position are calculated directly from the velocity vectors obtained using PIV. In order to determine the most appropriate value of bound circulation, a two-dimensional potential flow model is employed and flow fields are calculated for a range of values of bound circulation. In this way, the value of bound circulation is selected to give the best fit between the experimental velocity field and the potential flow field. Early in the trajectory, the value of bound circulation calculated using this potential flow method is in accordance with Kelvin's circulation theorem, but differs from the values predicted by Wagner's growth of bound circulation and the Kutta condition. Later the Kutta condition is established but the bound circulation remains small; most of the circulation is contained instead in the LEVs. The growth of wake circulation can be approximated by Wagner's circulation curve. Superimposing the non-circulatory lift, approximated from the potential flow model, and Wagner's lift curve gives a first-order approximation of the measured lift. Lift is generated by inertial effects and the slow buildup of circulation, which is contained in shed vortices rather than bound circulation. © 2013 Cambridge University Press.

Description: The ready availability of full-field velocity measurements in present-day experiments has kindled interest in using such data for force estimation, especially in situations where direct measurements are difficult. Among the methods proposed, a formulation based on impulse is attractive, for both practical and physical reasons. However, evaluation of the impulse requires a complete description of the vorticity field, and this is particularly hard to achieve in the important region close to a body surface. This paper presents a solution to the problem. The incomplete experimental-vorticity field is augmented by a vortex sheet on the body, with strength determined by the no-slip boundary condition. The impulse is then found from the sum of vortex-sheet and experimental contributions. Components of physical interest can straightforwardly be recognised; for example, the classical 'added mass' associated with fluid inertia is represented by an explicit term in the formulation for the vortex sheet. The method is implemented in the context of two-dimensional flat-plate flow, and tested on velocity-field data from a translating wing experiment. The results show that the vortex-sheet contribution is significant for the test data set. Furthermore, when it is included, good agreement with force-balance measurements is found. It is thus recommended that any impulse-based force calculation should correct for (likely) data incompleteness in this way. © 2017 Cambridge University Press.

Description: An experimental study of bluff bodies in confinement is presented. Two Reynolds matched rigs (pipe diameters: and) are used to derive a picture of the flow topology of the primary-shedding mode (Kármán vortex, mode-I). Confined bluff bodies create an additional spectral mode (mode-II). This is caused by the close coupling of the shedder blockage and the wall and is unique to the confined bluff-body problem. Under certain conditions, modes-I and II can interact, resulting in a lock-on, wherein the modes cease to exist at independent frequencies. The topological effects of mode interaction are demonstrated using flow visualisation. Furthermore, the scaling of mode-II is explored. The two experimental facilities span Reynolds numbers (based on the shedder diameter,) 〈![CDATA[$10^{4}〈Re-{d} and bulk Mach numbers 〈![CDATA[$0.02〈M-{b}. Bluff bodies with a constant blockage ratio (), forebody shape and various splitter-plate lengths () and thicknesses () are used. Results indicate that the flow topology changes substantially between short (〈![CDATA[$l) and long (d$]]〉) tailed geometries. Surface flow visualisation indicates that the primary vortex becomes anchored on the tail when (). This criterion prohibits the development of such a topology for short-tailed geometries. When mode interaction occurs, which it does exclusively in long-tailed cases, the tail-anchored vortex pattern is disrupted. The onset of mode-II occurs at approximately the same Reynolds number in both rigs, although the associated dimensionless frequency is principally a function of Mach number. Accordingly, mode interaction is avoided in the larger-scale rig, due to the increased separation of the modal frequencies. © 2019 The Author(s).

Description: SummarySodium chlorite increased the degradability of fibre from a range of mature grass forages inoluding barley straw by 40–50 digestibility units when comparisons were made using ground (1 mm sieve) material incubated either in vitro with cellulase or in nylon bags in the rumen. However, when fed to sheep, chlorite-treated barley straw was digested to a similar degree to untreated straw (49 and 57% respectively), but intake was significantly reduced (385 and 790 g/day respectively). The poor in vivo utilization of chlorite-treated straw coincided with high proportions of propionic to acetic acid in the rumen (0·85, cf. 0·28 with untreated feed) and absence of rumen fungi. Rumen pH and ammonia concentrations were not significantly different between diets. When incubated in nylon bags in the rumen of animals fed the corresponding diet, both untreated and treated straw (〈 1 mm) were well colonized with micro-organisms, as measured by cystine accumulation which showed peaks on the fibres after 24 and 72 h. While large numbers of fungal sporangia were observed after 24 h incubation on digesta from untreated forage, no fungi could be detected on the chlorite-treated material. Cystine accumulation on the untreated straw after 72 h was not associated with a secondary growth of fungi.Although barley straw chaff, ground (〈 1 mm) after treatment with chlorite, was highly degraded in nylon bags in the rumen and with cellulase in vitro, larger particles (1 cm) suspended in nylon bags in an animal fed chlorite-treated straw actually gained weight, probably due in part to the degree of microbial colonization.

Description: SummaryCell walls from mature stems of three tropical grass species (Digitaria decumbens (pangola), Setaria anceps (cv. Kazangula) and sugar cane), and temperate barley straw, were analysed for lignin, carbohydrate, and the maj or acyl groups ferulate, ρ-coumarate and acetate. Samples were incubated in nylon bags in the rumen of sheep in a 4 x 4 latin-square design, and rates of disappearance of cellulose, hemicellulose, xylose, arabinose, ferulate, ρ-coumarate and acetate were determined during 60 h incubation. Interspecies differences in cell-wall chemistry appeared largely in the variable degree of acylation with p-coumaric acid (1·0–3·3%) and acetate (0·5–3·6%), and the high glucose concentration in the hemicellulose from pangola (17%) and Setaria (9%). Barley had much lower concentrations of these components than the tropical species. After 24 h incubation, losses of cellulose and acyl groups were greatest from pangola, whereas hemicellulose and its major components xylose and arabinose were degraded to the greatest degree from barley straw. Setaria cell-wall components were generally more resistant to degradation than the other species. No relationship was found between the concentration of any cell-wall constituent and degradability measurements. Nor were changes in microbial population, indicated by measuring the accumulation of cystine on the fibres, related to the rate or degree of degradation of any of the measured cell-wall constituents. Lignin was fractionated with alkali into insoluble and soluble fractions. The latter (25–50% of original lignin) gave high interspecies correlations with the degradability of total hemicellulose and its component monosaccharides. It was concluded that variability in the biodegradability of the cell walls was more likely due to in situ structural features, such as cross-linking between polymers, than to the concentration of any particular cell-wall constituent.

Description: SUMMARYCell wall preparations from stems of four mature grass species, pangola grass, setaria, sugar cane and barley straw were incubated in nylon bags in sheep fitted with rumen cannulae and fed chopped pangola grass at hourly intervals. After varying incubation times D.M. loss, and incorporation of 35S into microbial cystine on the fibres, were measured. Pangola and barley straw were digested to a much greater extent (ca. 48 and 44%) than sugar cane and setaria (ca. 29 and 23% respectively) and digestion was still continuing after 60 h. With the exception of setaria, microbial colonization of the cell wall preparations peaked after 24 h incubation and then declined. In setaria only a small amount of [35S]cystine was measured, the level of which did not change appreciably after 18 h.After 24 h incubation, microbial colonization on pangola fibre was about three times that on barley straw and sugar cane. Only on pangola fibre did cystine accumulation, and its subsequent rapid decline, coincide with the development and detachment of fungal sporangia. There was no relationship between the extent of microbial colonization and D.M. loss from the fibres. Sulphur concentrations, both in the plant fibres and rumen fluid, could not explain the greater fungal growth on the pangola cell walls in preference to the other species.